A compound term combining the words "shee" (an alternate form of "shit") and "brah" (an alternate form of bro).
Used as an exclamation, but also refers to the word animal zebra, since the words sound alike. However, the association between Sheebra and an actual zebra are unclear.

Brian: Yo we have a story outline due on tuesday
Ryan: Sheebra
Brian: today's tuesday
Ryan: Sheebra
Brian: wtf is a sheebra
Ryan: lols you need to learn this shee, brah